The invention is in the field of electronic reproduction technology and is directed to a method for engraving printing cylinders in an electronic engraving machine, whereby at least two engraving lanes that lie next to one another in axial direction are engraved on a printing cylinder with a respective engraving element.
DE-C-25 087 34 already discloses an electronic engraving machine for engraving printing cylinders with an engraving element. The engraving element, having an engraving stylus controlled by an engraving control signal as a cutting tool, for example in the form of a diamond, moves in the axial direction along a rotating printing cylinder. The engraving stylus cuts a sequence of cups arranged in a printing raster into the generated surface of the printing cylinder. The engraving control signal is formed in an engraving amplifier by superimposition of a periodic raster signal, also referred to as vibration, with image signal values that represent the printing densities between “light” and “dark” to be reproduced. Whereas the raster signal effects an oscillating lifting motion of the engraving stylus for engraving the cups arranged in the printing raster, the image signal values determine the cut depths of the engraved cups in conformity with the tonal values to be reproduced.
For magazine printing, a plurality of strip-shaped cylinder regions called engraving lanes that lie axially next to one another must be simultaneously engraved with a respective engraving element on a printing cylinder or on the printing cylinders of a color set that are successively engraved in one engraving machine or, on the other hand, simultaneously engraved in a plurality of engraving machines. For example, the various printed pages of a print job are produced in the individual engraving lanes. The engraving control signals for the individual engraving elements are thereby generated in separate electronic units, referred to as engraving channels.
A pre-requisite for a good reproduction quality is that the engraved printing densities in the individual engraving lanes coincide, i.e. that what is referred to as a lane equality is achieved. Even when the individual engraving channels are electrically balanced, the engraving styli often exhibit different degrees of wear. The result is that cups having different geometrical dimensions or volumes are engraved in the individual engraving lanes, as a result of which disturbing differences in printing density occur in the engraving lanes. Worn engraving styli also generate cups with a rougher inside surface, as a result of which the ink acceptance behavior in the printing machine and, thus, the printing density are varied. Different printing densities in the engraving lanes can also be attributed to influences in the printing machine, for example when the pressing power between printing cylinder and cooperating printing cylinder varies in the axial direction or when the doctor blade with which excess ink is stripped off does not lie against the printing cylinder with the same tightness everywhere.
In order to achieve identical printing densities in the engraving lanes, engraving styli having the same degree of wear are currently sought insofar as possible for the engraving. As a precautionary measure, the engraving styli are also replaced by new engraving styli after a specific number of operating hours, this being relatively involved and expensive.
Even when new engraving styli are employed, density differences due to engraved areas that differ in size can soon arise in the individual engraving lanes as can, connected therewith, wear of the engraving styli that occurs with differing repetity. Different engraving properties such as the hardness of the material and the cutting behavior of the engraving stylus in the material—whereby the material is generally copper—can, for example, arise due to a non-uniform galvanization of the printing cylinder.
The differences in printing density in the engraving lanes can occur with respect to one printing density value or with respect to a printing density range and can differ in size for each engraving lane. Additionally, the engraving properties can change at the circumference of the printing cylinder, so that differences in printing density can also occur within an engraving lane.
For matching such differences in printing density, the engraved printing cylinder is currently chemically post-processed in practice in a time-consuming and work-intensive work process, particularly given high quality commands made of the printed products printed with the cylinder.